I'd rather loose a mark by grounding a wide sump guard, than not finish a trial having damaged my ignition / gears. It really depends on what sort of terrain your competing in.
There are many tank sealants ,https://www.caswelleurope.co.uk/ethanol-proof-fuel-tank-sealer/
How successful they would be on an old dirty tank is debatable. Personally, I would go for an alloy liner using the original as a cover.
You can replace the outer seal and o ring easily, think they are all the same but best to check https://www.motoswm.co.uk/shop/swm-rotax-kickstart-clutch-cover-oil-seal-kit/
There is another o ring on the other end of the shaft, which requires casings to be split to replace.
As above, refit it and tension as per spec. Then looking at the rear sprocket, at the point furthest back, pull it away from the sprocket, if there is any significance movement it probably needs replacing. What condition are you sprockets in ?
